The integration of telecommunications and data services leads to a series of requirements for the switching technology and networks. In order to be suitable for infotainment and commercial traffic at the same time, the networks used should have a high capacity, allow transmission in realtime, be reliable and guarantee a high degree of safety. In addition to this, a further condition is to keep costs as low as possible.
Previously, data services were to a large extent processed via IP networks (networks based on the Internet Protocol) that operate packet-oriented and connectionless at the IP level. Progress in router technology have led to the development of IP routers that, with a view to the capacity of the switched data traffic and delay times because of queues, basically allow telecommunications services and bandwidth-intensive services such as video-on-demand or video conferences in real time.
Difficulties are encountered in the case of a high rate of capacity utilization of the IP network for which the delay times increase exponentially or the excessive aggregation of data traffic on individual routes which then act as a bottleneck and limit the transmission rate.
Because of these difficulties exist, a high service quality generally referred to in the literature as quality-of-service (QoS) cannot be guaranteed to the desired extent for conventional IP networks.
Further developments aim at obtaining better information about the service quality without impinging on the lack of complexity and flexibility of the IP network.
The differentiated services (diff-derv) model is based on the observation that the best-effort handling of data packets in the IP network leads to the above-mentioned difficulties in guaranteeing service quality. Conventionally, data packets are transferred quickly and completely without guaranteeing the reliability and safety of the transmission. During high utilization or overloading of the network the service quality is impaired by delays or discarding of data packets.
The diff-serv concept aims at improving the service quality for services with high quality requirements by introducing service classes. In this context, the term CoS (class of service) model is often mentioned. The numbers 2474 and 2475 describe the diff-serv concept published by the IETF. The RFCs 2638 and 2998 deal with further aspects of the concept. Within the framework of the diff-serv concept, a DS (differentiated services) field in the IP header of the data packets prioritizes the packet traffic by setting the DSCP (DS code point) parameters. This prioritization takes place with a “per hop” resource allocation, i.e. the packets are treated differently for the nodes depending on the class of service specified by the DSCP parameters in the DS field. The expression per-hop behavior (PBH) is used in this context. For example, in the course of a PBH, priority is given to higher classes of service with regard to the arrangement and processing of queues in the case of nodes.
The central elements of a network based on the diff-serv concept are the DS subnetworks—in English often called the DS domain or single routing domain—and the DS boundary nodes. In many cases, a subnetwork conforms to the network of a service provider (service provider domain). For the DS boundary nodes a distinction is made between the DS ingress nodes and the DS egress nodes. Data packets reach a DS subnetwork via a DS ingress node and leave the subnetwork via a DS egress node. Therefore, a DS boundary node can then unite the functionality of a DS ingress node for incoming traffic and a DS egress node for outgoing traffic. The functionality of the DS boundary node includes selecting data packets according to the DSCP parameters and marking data packets by means of DSCP parameters. In addition, by means of devices for managing and conditioning the traffic, it is also possible to carry out control measures such as measuring data flows, distributing data packets to queues or rejecting data packets in the DS subnetwork. These control measures are often carried out in the DS ingress node or the DS egress node. Within the framework of traffic conditioning, data packets can be classified and compared to a traffic profile (e.g. bandwidth, resources) provided for the corresponding class of service. In the case of deviations from the traffic profile, measures such as arranging a queue or rejecting data packets can be carried out.
Typically, flows or connections are prioritized in a DS subnetwork. Prioritization takes place in a DS ingress node for the corresponding DS subnetwork, if required, by setting or changing the DSCP parameters. The core nodes of the DSCP parameters are read and assigned to the resources according to the prioritization. The DSCP parameters and resource assignment of the individual core nodes are interpreted separately from one another (per-hop behavior). For the egress nodes, changing the DSCP parameters is possibly canceled, i.e. the DSCP is reset to the original value. In this way, the DSCP can be adapted locally, i.e. depending on the subnetwork and its features.
The diff-serv concept avoids complex reservation procedures of routes or bandwidth and prioritizes the data traffic. When transmitting via several subnetworks, classes of service are specified by means of so-called service level agreements (SLA) for the entire transmission and converted by the individual subnetworks as described above within the framework of traffic conditioning. However, in practice temporary and/or local shortages, for example, by the aggregation of data traffic to individual routes also occurs. Usually, data packets with the same destination from the time they came together in a node, follow the same set route. For shortages the diff-serv concept sees to it that data packets with a low priority are first of all delayed or rejected. Therefore, the transmission quality for the high-priority data packets is improved, but quality standards e.g. for real-time transmission cannot be guaranteed. Within the framework of the diff-serv concept it would only be possible to guarantee transmission with QoS, i.e. a transmission for which specific quality statements are given and adhered to if the traffic profiles are adjusted with such a low utilization of the subnetwork that load peaks were catered for by reserve bandwidths. This is not usually undertaken for reasons of cost, i.e. because of the resulting low network utilization. For this reason, within the context of the diff-serv concept, reference is made to a CoS (class of service) approach rather than a QoS (quality of service) approach.